High performance coal combusting and chemical conversion equipment can utilize a coal extruder that advances coal from a hopper to a pressurized combustion chamber, or the like and which forceably extrudes the coal into the chamber. During the extrusion process, the coal is rapidly heated from room temperature to several hundred degrees centigrade such as 500.degree. centigrade. During such heating, the coal undergoes a great change in viscosity and bulk density, which is largely due to phase changes in the coal, including melting, vaporizing, polymerization and decomposition of certain coal components. The design of efficient coal extruders and other equipment that handles heated coal, can be greatly facilitated by knowledge about changes in the viscosity and bulk density of the particular coal which is to be used, as its temperature increases.
Typical prior art equipment for measuring viscosity includes a simple cylindrical chamber with a narrow opening at one end, and a simple piston which closely fits within the cylinder. The cylinder and piston could be heated to a high temperature such as 500.degree. C., and an unheated solid coal sample could be inserted in the cylinder. The coal sample cannot be substantially preheated, since this would drive off the volatiles, oxidize the coal, and make it difficult to handle. The piston can then be pressed with great force into the cylinder, to extrude the coal which is rising in temperature, while measurements are made of the outflow of coal from the narrow opening. This technique has many deficiencies, including the fact that it is difficult to seal in volatiles in the coal, which therefore can escape past the piston to yield higher viscosity measurements than would be encountered in a coal extruder wherein the gases were trapped in the coal being extruded.
Another problem is that the temperature of the coal sample rises only slowly towards the temperature of the cylinder and piston, due to poor conductive heat transfer between the walls of the cylinder and piston and the coal sample. As a result, the large mass of the sample which lies between the end wall of the cylinder and the piston, will be at a much lower temperature than the sample portion being extruded through the small hole in the cylinder, so that the large portion of the sample will resist piston movement and prevent the application of most of the force applied to the piston to the portion of the sample which is being extruded through the hole. These and other great differences between the environment of the coal extruder utilized with a combustion chamber, and a sample utilized in a prior art rheometer, have made it difficult to obtain accurate measurements of coal viscosity to facilitate the design of efficient coal handling equipment. Accordingly, in the prior art, measurements have typically been on scales of relative viscosity such as the dial divisions per minute of the Geisler scale for a particular coal sample. A rheometer which enabled the measurement of the viscosity and/or bulk density of coal or other materials that underwent phase change at elevated temperatures, in a manner more closely similar to situations encountered in actual coal handling equipment, would facilitate the design of more efficient coal handling equipment.